There are three methods of supporting the main conductor. Where the track is close to the side of the street, a bracket carries the conductor over the middle of the track, as shown at D. Fig. 3. Where the track is double and in the middle of the street. poles with double brackets, as shown at 11, are sometimes used. The third method, and that most commonly followed, is to place it pole on each side of the street, with a light cross wire strung between them at right angles to the length of the street. From this cross wire the insulating support for the main conductor is suspended. The supports are placed not more than 125 ft. apart. When the line is very long. the traffic heavy, or the grades are very severe, insulated feeder wires are used to supplement the main conductor, to which they are connected at proper points along the line. Fig. 4 shows the method of center-pole construction and trolley arrangement, as embodied in the Thomson-Houston electric road at Washington, D. C.
The Sprague System.—The introduction of the Sprague electric railway motor in 1886 constituted a distinct change in the method of propelling street cars by electricity, and was the beginning of a practice of operation which has become almost universal. The objects sought and accomplished in Mr. Sprague's method were to remove the motor from the car body, place it under the car. make positive connection between the machine and the axle, drive by gearing, and to allow independent movement of the axles, and to preserve elasticity in mechanical connections. Independent driving and positive gearing became cardinal prin ciples, and this necessitated a yielding and preferably cushioned support. To these ends one motor is centered upon each axle, and, to allow the required freedom of motion and at the same time to preserve perfect parallelism in the meshing of the gears, and also for taking part of the weight of the motor off the body of the axle and to throw it onto the journals, one end of the motor is supported by double compression springs, playing upon a loosely sup ported bolt, which is supported from a cross girder in the bottom of the car or on cross beams supported directly on the side framing of the truck or on equalizing bars carried by the axle boxes. The motors are then, so to speak, weighed or flexibly supported from the car body, and the motion of the armatures being transmitted to the axles through intermediate gearing of compact form and great strength, whenever the axles are in motion there is a spring touch of the pinions upon the gears. Earring friction, a single pound press ure exerted in either direc tion would lift or depress the motor a slight amount, and no matter how sudden the strain, whether because of a variation of load or speed, or a reversal of direction of rotation, the motor yields to it so as to make the pressure on the gears a progressive one. This support, allowing the armature
to have an angular movement several times that of the motor around its axle, makes the strain on the armature much less also. The motors can be carried in the middle space be tween the axles, or external to them. although the former method is preferable. By detach ing the flexible suspension, the motors con be swung around the axle or allowed to hang from it over a pit, where they can be carefully inspected or cleaned.
Mr. Sprague has devised several forms of motors, double and single geared and gearless, to carryout these general prin ciples. The first put into use wasdesigned for compactness, lightness, and high speed on levels. This was a single geared machine, and the first one was used on experiments on the New York elevated railroad in 1886. The ma chine (Figs. 5 and 6) consists, in brief, of two curved field magnets bolted to two pole pieces, the whole carried by brackets on each side, which brackets center on the axle and also carry the armature. The free end of the motor is supported by a bolt and double-acting springs from the transoms of the truck. In this motor the gear reduc tion is about five to one, and the driving from both ends into gears bolted to the axle. The pinions on the armature shaft are, by a very simple and ingenious construction, set so that the one is half a tooth in advance of the other. For high speed and heavy work this method is very efficient.
In a later type of motors (Fig. 7) the D • shaped magnet has been adopted with a com plete wrought-iron magnetic circuit, the gear reduction is double, the armature pinion mesh ing with the larger of two gears on an intermediate countershaft carried on brackets which engage the main axle, the pillion on the intermediate in turn meshing into a single gear on the axle. This machine has been very widely adopted, and is the plan of double-geared machines now generally employed. The suspension of the free end of the motor has been sometimes made from the car body, but it is more generally carried by equalizing bars yield ingly supported on the axle boxes, so that the resilieaaee and size of the springs that support the car body is undisturbed. The ratio of reduction in gearing is dependent upon the size of the motor and the character of the work required of it. It is essential, on account of the limited space and the necessity of driving both axles, to have two machines, one geared to each axle and independently mounted; the free ends of the machines are inboard, that is, the entire motor equipment is in the space between the axles.